Waving Grass Behaves Differently When Replacing Built-In Version with the Same Shader

The grass on my terrain is a detail mesh with render mode set to "Grass." The grass waves in the wind as expected.

I wanted to tweak the built-in grass shader (Hidden/TerrainEngine/Details/WavingDoublePass) because it was too shiny, so I downloaded the built-in shader code from the Unity Archive. Version 5.1.1 to match my Unity version. Then I moved the WavingGrass script into my project.

Before making any modifications to the newly added shader script, I tested it. I found that instead of each grass mesh bending the wind realistically as before, each entire mesh now slides around on the ground.

I would have expected the grass to behave exactly the same as before since the shader that I downloaded should be identical to the built-in one that it replaced. I am wondering why the grass is behaving differently.

Thanks!

EDIT: I painted the vertices using a script that came with Advanced Terrain Shader (from the asset store), and it seems to have fixed the problem. I never figured out why replacing the script changed things though.